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I grabbed an LED and then scraped an aluminum plate with a pin, and it turned on at some points, at first I thought it was electrified but it wasn't, then I tried with another aluminum plate and it also happened.

Later I tried the same thing with a cable connected to my computer's microphone jack and recorded the spectrum:

The first image is zoomed in, and the background waves are noise from the mains

What is causing this?

enter image description here

enter image description here

Marcus Müller
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Sartem Cacartem
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    Well done! Now did you have a question? – Oldfart Nov 18 '19 at 14:21
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    This is awesome - love the exploratory nature ! As @Oldfart mentioned, you did not ask a question and only stated what you observed. This 'question' will be closed if there is no question to answer. – efox29 Nov 18 '19 at 14:23
  • i forgot the question it is the cause of the effect – Sartem Cacartem Nov 18 '19 at 14:37
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    It is likely the [triboelectric effect](https://en.wikipedia.org/wiki/Triboelectric_effect), however I am not certain of this. – rdtsc Nov 18 '19 at 15:35
  • @rdtsc Does that even happen for metal on metal? – DKNguyen Nov 18 '19 at 16:01
  • Were the plate and the pin connected to the LED and the microphone input at all? In what way? – user253751 Nov 18 '19 at 16:01
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    @DKNguyen apparently it [does for aluminum](https://en.wikipedia.org/wiki/Triboelectric_effect#Triboelectric_series), but it is only slightly positively-charged. – rdtsc Nov 18 '19 at 16:25
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    In the LED experiment i was touching one pin and the other pin was scraping the aluminum, it seems that the led turned on because the effect generated AC and i have some capacitance, in the microphone experiment it was the same but the capacitance was from the sound card – Sartem Cacartem Nov 18 '19 at 17:15
  • that didnt answer the question, only explained why the current could pass by just one cable – Sartem Cacartem Dec 02 '19 at 03:33
  • You were probably picking up the large static charge, while the aluminum isn't...and therefore you discharge to the aluminum through the LED. It's surprising how little current is needed to visibly illuminate one. – Cristobol Polychronopolis Dec 31 '19 at 13:19
  • yes i could illuminate a 3mm led using 5 microamps – Sartem Cacartem Dec 31 '19 at 17:52
  • So, this had to be a 2in aluminum plate sitting on a wooden table, with no fingers touching? Or maybe a three foot aluminum plate bolted to the front of an equipment rack? You don't say, but "aluminum plate" could actually be a sort of Earth-connection, with nothing to do with aluminum (unless it's a small piece, and suspended by an insulator such as wood!) – wbeaty Feb 22 '22 at 23:52

1 Answers1

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There are two factors at play here.

The first thing to understand is that all aluminum has a thin clear layer of Aluminum Oxide on it, and when you scrape it away exposing pure aluminum it will very quickly reform, so it is almost always there.

Aluminum Oxide has a significant piezoelectric effect, strong enough that it is often used in science experiments specifically for this effect. So as you are scraping across the top of the metal you are breaking apart the bonds in the Aluminum oxide and as you do this it causes little jolts of voltage. You can even see this effect if you crush a sapphire in a dark room with a hammer, little sparks will form momentarily. A sapphire is just Aluminum Oxide more or less.

The piezoelectric effect I just described is likely the dominate effect.

There is a second effect that would be more than strong enough to show up as well, though I suspect less pronounced, that is what is called the Galvani potential of the two metals. When you have two dissimilar metals and they come in contact for a short time a small current flows and there is a difference in potential between them. In fact, the amount of voltage difference is determined not just by the metals but the temperature. This is exactly how thermocouples work, they are just two wires o different metals touching, we measure the voltage across them and this translates to a temperature.

Since the metal of your probe is not aluminum when it comes in contact with the aluminum a small voltage is produced. As you scrape over the rough surface the connection between your probe and the underlying aluminum isnt making great contact (remember there is a non-conductive oxide layer you are cutting though). On a microscopic level your probe is bouncing up and down making momentary contact with the aluminum as it scratches across very quickly. So this constant scratchiness between the two metals is likely causing the normal potential difference to vary and adding some noise to your plot as well.

Jeffrey Phillips Freeman
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